1. Field of the invention
This invention relates to a disc apparatus used for reproducing data recorded on a disc-shaped recording medium.
2. Description of Related Art
In a disc apparatus used as a peripheral equipment of a computer or as a disc apparatus for recording/reproducing picture data, a phase-change optical disc or a magneto-optical disc (rewritable disc) i s used, which i s a disc-shaped recording medium capable of recording a large capacity data to high density and of repeatedly rewriting the data. The phase-change optical disc is such a disc in which data is recorded by phase transition of the recording film between the crystal state and the amorphous state by laser heating and in which the recorded data is read out by exploiting the fact that light reflectivity of the recording film differs depending on these states. On the other hand, a magneto-optical disc is such a disc in which coercivity of a recording film is transiently lowered by laser heating for magnetically recording data and in which the recorded data is read out by exploiting the fact that the angle of polarization of the reflected light differs depending on the magnetized state of the recording film.
With the above-described disc-shaped recording medium, referred to hereinafter as disc, the data writing beginning position and the data writing terminating position are determined by the recording format. Thus, the data rewriting accompanied by laser heating occurs repeatedly at a specified position of the disc where damage to the recording film is liable to be caused. This damage is caused by thermal fluidization of the recording film due to elevated temperature brought about by laser heating. This phenomenon, termed material flow, presents a problem in particular with the phase-change optical disc.
In the above area subjected to the phenomenon of material flow, data errors are liable to be produced during recording/reproduction, thus lowering reliability. If damage to the recording film proceeds further, data recording/reproduction becomes infeasible. In particular, since the data writing beginning position is an area where the sync pattern as the synchronization signal is written, the phase locked loop is disturbed to render it impossible to keep synchronization or the synchronization pattern. Moreover, the Sync pattern is detected by mistake to render re-synchronization time-consuming.